In order to ensure that the supply of contents to the packaging material tube takes place at the desired rate, i.e. a rate which ensures that the quantity of contents packed in each individual packaging container is replenished so that the level of contents in the tube is kept substantially constant, packaging machines of the above-mentioned type are normally provided with some form of level maintenance device which senses the level of the contents and controls the supply of contents. Prior art level maintenance devices of this type utilize a float located in the material tube, the position of the float being sensed and utilized for controlling, via a valve or pump, the supply of contents to a filler pipe extending downwards through the upper, open end of the packaging material tube. The sensing of the position of the float can-take place either with the aid of devices disposed within the packaging material tube or with the aid of external sensing devices which electrically control a contents supply valve (see, for example European Patent EP 322.001).
The prior art types of level maintenance devices which have hitherto been employed in the art all make use of one or more components disposed in the packaging material tube, for example the above-mentioned float, ultrasonic sensors, or level probes. However, in packaging machines of the above-mentioned type (which may possibly also be called upon to operate under aseptic, i.e. sterile, conditions), reasons of hygiene dictate the desirability of reducing the number of mechanical parts located inside the packaging material tube. With a minimum of mechanical parts inside the packaging material tube, i.e. in direct contact with the contents, cleaning of the packaging machine will be considerably facilitated, which ensures that a high standard of hygiene can be maintained. Thus, it is desirable in the art to devise a method for registering the level of the contents in this type of packaging machine, the method making it possible to register the level of contents from the outside of the packaging material tube and without direct contact with the contents located inside the packaging material tube.